Production of stainless steel



Patented Dec. 1, 1942 UNITED STATES PATENT OFFICE PRODUCTION OF STAINLESS STEEL Alexander L. Feild, Towson, Md.,' assignor to Rustless Iron and Steel Corporation, a corpora- .tion of Delaware No Drawing. Application December 30, 1940,

Serial No. 372,400

Claims. (01. 75-127) My invention relates to the production of stainless steel.

Among the objects of my invention is the production of stainless steel in a simple, efllcient and thoroughly practical manner, employing available and inexpensive raw materials and using known and available melting equipment and emat this point that" the stainless steels are alloy irons and steels containingchromium in the amount of from about 12% up to about 35%. Other alloying ingredients, such as silicon, manganese, copper, nickel, cobalt, tungsten, molybdenum or titanium, vanadium, tantalum, columbium, or sulphur, phosphorus, selenium, tellurium or arsenic may be present, as desired, to achieve well-known special properties. The amounts of the austenite-forming ingredients, nickel and manganese, may be sufficient to render the resultant metal austenitic. The carbon content or-' dinarily is low, .03% to as is also the nitrogen content, .005%- to 935%. Higher carbon and nitrogen contents may behad where desired, as

in the hardenable grades of stainless steel, either of the martensitic or austenitic types.

In the production of stainless steel, in accordance with known direct ore methods, large quantities of chrome ore are employed-as a substantial source of chromium. This ore is highly refractory and fuses only with the greatest of difllculty. Furthermore,-theore necessary to achieve a desired chromium addition presents a considerable volume. In fact. the volume of ore required for an appreciable chromium addition is such that the tanning weight of metalis severely limited in order that the slag volumes encountered may not become unmanageable. The slag is further increased in amount by virtue of the operation of a silicon-containing reducing agent, as well as the large quantities of lime which. are employed in the reduction of the ore,

The above-mentioned difliculties of a practical operating nature arise from the large volume of rosilicon reduction process. There are three sources of slag formation. In the first place,

chromite, the only commercial ore of chromium,

" contains a large proportion of irreducible gangue which is composed almost entirely of the refractory oxides, magnesia (MgO),'alumina (A1203) and silica (S102). Thisgangue material, being insoluble in the metal bath, forms a part of the supernatant slag. In the second place, the silicon which is the active reducing agent combines with the iron and chromium oxides in the chrome ore to form silica ($102) as a by-product. This by-product silica likewise comprises a large proportion of the slag volume or weight. Finally,

the slag further increases as a result of theaddition of a large quantity of lime added as aneutralizing or basic fluxing agent, all in accordance with well understood principles.

One object of my invention is the manufacture of stainless steel ingots, castings, or the like by means of a novel, economical and practical process which utilizes the maximum quantity of chrome ore consistent with the best steel making practice and produces at the same time a minimum volume of slag for a maximum amount of metal.

; may be employed. Such a furnace is rated threephase, 60 cycle, K. V. A. at 110 to 250 volts. The furnace preferably is lined with a chromite brick, conveniently to a height just above the slag line.

The raw materials, comprising steel scrap and chrome ore, are charged into the furnace and power is applied to the furnace electrodes. The chargemelts down, forming a bath of metal and an overlying blanket of slag. This slag contains the oxides of iron and chromium. In addition,

it contains alumina, magnesia and other materials forming a gangue. The ore is highly refractory. In order that proper furnacing conditions may be maintained during the later stages of my process, I have found that a major portion of the ore must be melted along with the steel scrap.

Only a minor portion may be added during the slag unavoidably formed during the ordinary fercut is achieved when the silicon content ranges ing chromium content is from about 45% to 35%. The balance of the alloy is iron. The ratio of chromium to silicon contents ranges from about .7 in the latter case to 1.1, in the former. Good results are had where the ratio is from .6 to 1.2. A grade of ferrochrome silicon having this ratio is necessary to the practice of my invention. I find that, for some reason unknown to me, such a reducing agent is better taken into the metal bath and effects a more efficient reduction of the oxides present in the ore than when there is employed a ferrosilicon of corresponding silicon content.

A higher silicon content is not generally desired because of the corresponding lowering of the chromium content. Similarly, a lower silicon content with an increased chromium content ordinarily is not permissible because the excessive dilution is inclined to render the alloy ineffective as a reducmg agent. Best results from a practical standpoint appear where the 50% silicon grade of alloy is used giving a chromium to silicon ratio of about .7.

The quantity of ferrochrome silicon added is chemically in excess of that theoretically required to reduce the oxides of iron. and chromium present. I find that the introduction of at least an equivalent amount of the reducing agent, and

preferably an excess of this ingredient, is essential to the realization of consistently satisfactory results; the amount of the. reducing agent ordinarily ranging from to in excess of that theoretically required as more particularly noted hereinafter. This relationship is necessary to the preservation of proper furnacing conditions as well as to the production of clean metal where ferrochrome silicon is employed. Moreover, high recovery from the chrome ore, efliciency of manufacture, and ease of slag removal and disposition require that the ferro-chrome silicon be employed in excess of the stoichiometric relationship with the reducible oxides present.

Along with the ferrochrome silicon, there is charged into the furnace substantial quantities of lime. This addition maintains a basic condition during the reducing period and effectively prevents silicon contamination of the heat of metal being produced. The amount of this addition is in excess of that necessary to combine with the silica formed during the reducing operation.

Ordinarily the limeand ferrochrome-silicon are initially mixed on the floor of the melt-shop and charged into the furnace as a mixture. This procedure, of course, may be departed from where desired. The mixture of lime and ferrochrome silicon is charged into the furnace conveniently in a number of successive batches. Any one batch preferably is permitted to react with the oxides of iron and chromium present in the slag before its successor is introduced. The slag which originally is hard, thick and viscous, becomes more and more fluid as the reducing action proceeds and it loses its content of chromicoxide,

In addition, its color changes from black or dark grey to a light grey somewhat greenish when allowed to cool.

The addition of ferrochrome silicon is continued until two conditions are satisfied. One, that the metallic values present in the slag are recovered, and two, that the desired chromium content has been introduced into the 'metal bath; the chromium coming from the oxides of chromium present in the slag and directly from the 2,330,991 from about 40% to The correspondreducing agents may be employed to achieve'a complete recovery of the metallic values present in the slag, without at the same time giving a metal of an unnecessarily high chromium content, a balance'must be preserved between the quality and amount of ferrochrome silicon used. A factor in striking this balance is the desire of obtaining a maximum weight of tapped metal for the capacity ofthe furnace used. Another factor is the comparative cost of chrome ore and ferrochrome silicon as sources of chromium. Other like factors of recovery and furnace practice also are involved.

When the usual grades of chrome ore are used as a source of chromium, those having a chromium content of about 50% chromium oxide and 15% ferrous oxide, I have determined a certain relationship between the raw materials employed. In accordance with my invention, the weight of ferrochrome silicon employed ranges from about one-half to one-quarter of the weight of chrome ore used for the range of chromium to silicon ratios set out above. Best results are had with the 50% silicon grade of ferrochrome silicon (chromium to silicon ratio of .7%) and where the weight of this ferrochrome silicon amounts to about one-third of that of the chrome ore used.

The following specific example will further i1- lustrate the process. is charged 16,266 lbs. of ordinary steel scrap together with 7,918 lbs. of chrome ore analyzing 48% chromic oxide (CrzOa) and 13.5% iron oxide (FeO) and the mixture is heated to form a slagmetal bath, the diflicultly fusible ore resting as a pasty, crusty, or partly fluid mass above the heavier molten steel below. When the steel scrap is of a bulky nature a portion of the total amount may be charged into the furnace at intervalsduring the melting period. All of the chrome ore is preferably charged into the furnace along with the steel scrap at the outset of the heat and during the melting period, the exact mannerin which such ore is charged depending upon a number of variable operating factors wellknown to those skilled in the art. By such procedure the chrome ore slag is brought to a high temperature before the reducing period about to be described is begun, thereby increasing the exothermic effect of the ferrochrome-silicon re ducing agent. However, I may, when I so desire, charge a minor fraction of the chrome ore along with the lime and reducing agent.

When the above-described slag-metal bath is thoroughly prepared and at the proper steelmaking temperature, 2,780 lbs. of crushed ferrochrome silicon, analyzing approximately 50% reducing alloy addition. In order that sufficient silicon and 35% chromium, is charged into the furnace with 5,560 lbs. of lime (CaO). Reduction of the chromic and iron oxides in the slag and the incorporation of the lime therein is then permitted to proceed under favorable fumacing conditions until the original chrome ore slag changes to a fluid basic finishing slag composed principally of the silicates and alumina-silicates of lime and magnesia and containing only rela-- tively small percentages of the oxides of chromium and iron. This slag is then removed and a basic finishing slag-of standard character and quantity is prepared. When the metal bath is ready for tapping, final adjustments in silicon and manganese contents and final deoxidizing additions are made. The heat is then tapped to yield 20,000 lbs. of stainless steel ingots analyzing .08% carbon and 17% chromium.

Onto the furnace hearth volume.

2,808,991 v The weightof finishing slag produced in the lbs. of chrome ore is charged into the furnace the quantity or weight of gangue amounts to 3,048 lbs. Again, the 2,780 lbs. of ferrochromesilicon reducing agent, analyzing 50% silicon, contains a quantity of silicon amounting to 1,390 lbs. which during its reaction with the oxides of chromium and iron is oxidized to silica ($102). The stoichiometric ratio of silica (SiOz) to silicon is equal to 2.14; hence, there is produced 1,390x2.14, or 2,975- lbs. of silica (S102) which enters the slag. The lime (CaO) amounting to 5,560 lbs. remains entirely in the slag. The slag weight, therefore, is equal to the sum of the weights of the ore gangue, silica, and lime given above which are, respectively, 3,048, 2,975 and 5,560 lbs., or a total of 11,583 lbs. This slag weight is the minimum, or calculated weight. In actual practice, of course, there is always, as is well known in the art, a certain increase in slag weight above the calculated weight 'due to wear and tear on the furnace refractories. V

A heat of the same weight made from chrome ore of the same analysis by ordinary ferrosilicon reduction required 10,914 lbs. of ore and calculations similar to the. above show that the sla volume would amount to 15,934 lbs. (and is made up of 4,196 lbs. of gangue, 4,090 lbs. of silica (SiOz) derived from ferrosilicon, and 7,648 lbs. of lime (CaO)). The decrease in slag volume, therefore, effected by the use of the process which comprises the present invention is equal to 15,- 934-11,583 or 4,351 lbs. This is equal to a reduction of 27.3% in slag weight and the same reduction in slag volume.

Taking into account the'relative density or specific gravity of steel and slag the saving in space or volume within the electric furnace hearth is equal to the volume of about 12,000 lbs. of steel. For any given furnace larger heats of steel product can be tapped and a greater production obtained.

My new process enables larger heats of steel to be made in a furnace of any selected hearth at a lower cost; and, when as in my preferred practice, an electric-arc steel melting furnace is employed, it also enables a greater length of arc for any given voltage, thereby diminishing materially the possibility of contamination of the metal by carbon from the furnace electrode.

Other obvious advantages of the new process are chrome silicon reducing agent which, because of its high silicon content, is low in carbon.

Whereas the present process. is particularly 1 adapted to the eflicient and economical production of stainless steel under conditions where little or no stainless steel; scrap is available, it may be employed of course with stainless steel scrap as a part of the .total steel scrap charge. Likewise, a certain amount of 'roll scale, hammer scale, or other oxide of iron, may be included in the charge of ingredients melted where stronger oxidizing conditions of melting are desired. Simthe long reducing period may be utilized to melt further quantities of chrome ore. In this embodiment of my invention a part of the ore is added with the initial charge and part with the ferrochrome silicon and lime. Probably best results are had wherethe chrome ore is added partly with the initial'charge, partly during the melting of this charge, .and partly during the reducing period, preferably with the reducing This effects directly a relatively large agent.

Thus. it will be seen that there is provided in this invention, a process of ,producing stainless steel in which the many objects hereinbefore noted, together with many thoroughly practical advantages, is successfully achieved. It will be seen that. the process is simple,. direct and eflicient; that it makes use of the least expensive raw-materials to achieve a desired chromium content; that it permits a maximum tapping weight of inetal, consistent with the use of inexpensive raw materials, for a given investment of fumacing equipment; and that it ofiers a flexibility in melting practice, permitting the use of certain quantities of stainless steel scrap which may be available around a melt-shop without, however, relying upon such scrap as a substantial source of chromium.

While as illustrative of the practice of my in- .vention, the production of the 17% chromium grade of ferritic stainless steel is given, it will be understood that steel making practice in accordance with the teachings of my invention is applicable to all of the stainless steels, including the ferritic, martensitic and chromium-nickel the savings effected by virtue of the smaller quantity of lime required per ton of ingots an'd the decreased expense of. slag handling and disaustenitic grades.- Likewise, while as illustrative, of my invention, the production of a ferritic type of stainless steel is described, the teachings of my invention are applicable to the production of a stainless steel containing considerable quantitles of other alloying-elements, such as, manganese, which are obtainable direct from the ores using silicon as the reducing ingredient. Where a manganese content is desired, say 2% to 20%, this may be achieved through a reduction of manganese ore. In such practice, the manganese ore is charged into the furnace along with the chrome ore or at a later period as dedesired.

As many possible embodiments may be made of my'invention, and asma'ny changes may be made in the embodiments hereinbefore set forth,-

herein is to be interpreted as illustrative, and not in a limiting sense.

I claim:

1. In the production, of stainless steel, the

art which includes, melting down a charge of silicon having a chromium to silicon ratio of from about .6 to 1.2, the total weight of said reducing agent being approximately to A of the weight of chrome ore employed and at least 10% in excess of that necessary to combine with the ore, whereby the oxides of chromium and iron of the slag are reduced, the metallic values gravitating into the metal bath, and clean, sound metal of a desired high chromium content and low carbon content is economically achieved with a minimum carbon pick-up during the reduction of the oxides and with a substantial portion of the chromium coming from the chrome ore and yet without encountering unmanageable volumes of slag.

2. In the production of stainless steel, the art which includes, melting down a charge of ingredients comprising steel scrap and chrome ore, giving a ferrous metal bath of a carbon content substantially lower than that of the charge and an overlying slag containing the oxides of iron and chromium, and'adding thereto burnt lime and a reducing agent of ferrochrome silicon having a chromium to silicon ratio of about .7, the total weight of said reducing agent being approximately of the weight of chrome ore employed, and 10% to 25% in excess of that necessary to chemically combine with the ore with the lime also in excess of the available silica, whereby the oxides of iron and chromium of the slag are reduced, the metallic values gravitating into the metal bath, and clean, sound metal of a desired chromium content and low carbon and silicon contents is economically achieved with a minimum carbon pick-up during the reduction of the oxides and with a substantial portion of the chromium coming from the chrome ore and yet without encountering unmanageable volumes of slag.-

3. In the production of stainless steel, the art which includes, melting down a charge of ingredients comprising steel scrap and chrome ore, giving a ferrous metal bath of low carbon content and an overlying slag containing the oxides or iron and chromium, and adding thereto an additional but minor quantity of chrome ore and a reducing agent of ferrochrome silicon having a chromium to silicon ratio of from about .6 to 1.2, the total weight of said reducing agent being approximately to A of the total weight of chrome ore employed and in excess of that necessary to chemically combine with the ore, whereby the oxides of chromium and iron of the slag are reduced, the metallic values gravitating into the metal bath, and clean, sound metal of a desired chromium content and low carbon content is economically achieved with a minimum carbon pick-up during the reduction of the oxides and with a substantial portion of the chromium coming from the chrome ore and yet without encountering unmanageable volumes of slag.

4. In the production of stainless steel, the art which includes, melting down a charge of ingredients comprising steel scrap, iron oxide, and chrome ore, giving a ferrous metal bath having a carbon content appreciably lower than that of the charge and an overlying slag containing the oxides of iron and chromium, and adding thereto an additional but minor quantity of chrome ore, burnt lime, and a reducing agent of ferrochrome silicon, said reducing agent being in excess of that theoretically necessary to chemically combine with the reducible oxides present and'having a chromium to silicon ratio of from about .6 to 1.2,the total weight of said reducing agent being from about to A, of that of the chrome ore employed and in excess of that chemically necessary to combine with the ore, with the lime also in excess of the available silica, whereby the oxides of chromium and iron of the slag are reduced, the metallic values gravitating into the metal bath and clean, sound metal of a desired chromium content and low carbon content is economically achieved with a minimum carbon pick-up during the reduction of the oxides and with a substantial portion of the chromium coming from the chrome ore and'yet without encountering unmanageable volumes of slag.

5. In the production of stainless steel, the art which includes, melting down a charge of ingredients comprising chrome ore, iron oxide and one or more of the class consisting of high car-.

bon steel scrap, low carbon steel scrap and stainless steel scrap, giving a ferrous metal 'bathhaving a carbon content appreciably lower than that of the charge and an overlying slag containing the oxides of iron and chromium, and

carbon content is economically achieved with a minimum carbon pick-up during the reduction of the oxides and with a substantial portion of the chromium coming from the chrome ore and yet without encountering unmanageable volumes of slag. ALEXANDER L. FEILD. 

